The invention relates to an electrical control device with crosstalk correction and to its application to magnetic write/read heads.
It can be applied to a source or a matrix of electrically controllable devices such as electromagnetic devices (magnetic heads for example) or optical devices to compensate for the crosstalk that may exist between neighboring devices.
In the case of magnetic write/read heads, the invention consists of a physical and passive device associated with an arrangement of elements enabling electrical compensation for the magnetic crosstalk that exists in the component for multitrack writing on magnetic media.
This device replaces a set of passive components (resistors and capacitors associated with their connection equipment) positioned between the electronic circuitry used to generate the write currents and the coils of the writing component.
The invention is applicable in the context of a system for the writing and reading of data elements on magnetic media where the physical format for writing data elements on tape is of the parallel multitrack type as shown in FIG. 1.
When these tracks are very close to one another, the component used for writing according to this format shows crosstalk in writing that becomes detrimental to the quality of the data elements written.
Magnetic crosstalk in writing results in a condition where the data elements normally designed for a track J are written partially on the neighboring tracks J-1 and J+1.
For example, a magnetic multitrack recorder has a large number of magnetic heads (1024 for example). This makes it possible to write 1024 tracks on a tape with a height of 12.54 mm. A device such as this has a matrix structure. The 1024 magnetic gaps are distributed in a parallelogram 16 columns long and 64 rows high.
The geometrical form of this parallelogram (essentially the angle) is carefully chosen to achieve the writing on the 1024 tracks on the magnetic tape adjacently without inter-track overlapping.
Each column and each row has an electrically addressed coil. In nominal operation, the currents flowing through the columns and the rows do not generate a field sufficient for writing on tape at the position of the gaps that correspond to them. By contrast, at the intersection of a column and a row, the additive action of the currents enables the generation, in the corresponding gap, of a field that is sufficient for the writing of a data element on tape.
In reality, when a column (or line) J is electrically addressed by a current +I, a magnetic field B proportional to this current appears in all the gaps of the column (or of the row) that is excited:
B=k.times.I, where k represents the efficiency of the write head.
Owing to the leakages from the magnetic circuits of the component, a parasitic field of crosstalk Bd which is a positive proportion of B also appears at the gaps located in the neighboring columns.
Bd=kd.times.B where kd represents the coefficient of crosstalk between columns of the write head.
When, in nominal operation, these fields of crosstalk that exist for all the columns and rows of the head get superimposed on those created by the currents, any track J is partially written on with the signals of the physically neighboring tracks.
Crosstalk in writing is very harmful to the quality of the channel because, during the re-reading, the data elements are stained with a noise correlated with the neighboring tracks which causes substantial deterioration in the signal-to-noise ratio.
Since the process that gives rise to crosstalk in the component is linear and reciprocal (Bd=kd.times.k.times.I), it is possible to cancel the field Bd on the columns J-1 and J+1 by sending a current -kd.times.I with reverse polarity into these columns during the addressing of the column J by the current I (see FIG. 2).
From an electrical point of view, this approach is complicated because it is necessary, in a synchronous manner, to generate currents having opposite polarities. Hence it is necessary to have symmetrical and linear control circuits since the current is proportional to kd.
This approach cannot be envisaged for reasons of power dissipation, for the number of control circuits needed is high (I lines+J columns). It is therefore simplified two-state control circuits (0 volt.fwdarw.+V volt) that supply the coil through a resistor.
FIG. 3 shows the electrical assembly used. The capacitor C, having a low value, is designed to increase the build-up time of the current in the coils. The common point of the coils fixed at +V2 enables the creation of the currents +I and -I from one and the same voltage.
The constraint on the nature of the control circuits therefore dictates another approach that enables the correction of magnetic crosstalk in keeping with the requirements as regards polarity.
The French patent application No. 92 15472 describes a device providing for coupling resistors to compensate for crosstalk.
In the basic configuration, all the coils of the component have an identical magnetic face, i.e. a current +I would give rise to a field +B=+k.times.I in all the gaps. Under these conditions, the correction current remains Id=-kd.times.I with the opposite sign.
In alternating the magnetic phase of one in every two coils throughout the component, hence with one coil wound in one direction and then the next one in the opposite direction, it is also necessary to alternate the sign of the current to generate a magnetic field with the same sign on all the gaps.
This principle is applied to all the rows and columns of the components. It will be observed, in FIGS. 4a to 4d, that an electrical information element +1 truly gives a magnetic field +B in the gap.
In this configuration, the crosstalk created by the coil J is corrected by the injection, into J-1 and J+1, of the current that gives rise to a field that is the reverse of the crosstalk field at J-1 and J+1. The reversal of magnetic polarity from one coil to another determines a correction current at J-1 and J+1 having the same sign as the current in J since the phase opposition is achieved by wiring.
The correction of crosstalk of the component can then be done solely with passive components as shown in FIG. 5. The crosstalk correction resistors Rd shunt a part kd of the current of J to J-1 and J+1 and give rise to the magnetic field which cancels the parasitic field.
Should a symmetrical control circuit be made for reasons of build-up time of the current in an inductive impedance, the number of components is doubled. The diagram of such a circuit is shown in FIG. 6.
In the case of a matrix type magnetic head with 1024 writing gaps distributed over 16 columns and 64 rows, the electrical assembly uses 64 simple control circuits for the rows and 16 control circuits in a bridge arrangement for the column. This configuration requires:
64+2.times.16=96 control circuits PA1 64=2.times.16=96 resistors for the control of the current of the coils PA1 2.times.64+4.times.16=192 crosstalk correction resistors. PA1 a first set of resistive coupling elements each connecting one input of an even-order resistive limiter element to an output of an odd-order neighboring resistive limiter element; PA1 a second set of resistive elements insulated from the first elements each connecting an output of an even-order resistive limiter element to an input of an odd-order resistive limiter element.
The writing component requires a network of 288 resistors in order to work accurately.